Structural elucidation and long-term stability of synthesized NADES: A detailed physicochemical analysis

• 20 different NADES, with a polarity range from 0.34 to 1.29, were synthesized. • The eutectic formation for NADES was elucidated by NMR and FT-IR. • Long-term stability (12 months) of NADES was demonstrated. • Dilution studies were conducted and identified that 30 % of water is optimal. • Spectral variability correlated with water content using PLSR modelling. The use of sustainable solvents is gaining momentum and contributing to the development of more environmentally friendly extraction methods. However, there is a lack of solvents that can cover a wide range of polarities, as is the case with conventional solvents. Therefore, this study focuses on the preparation and comprehensive characterization of different Natural Deep Eutectic Solvents (NADES) encompassing both hydrophilic and hydrophobic types. NADES were prepared through the heating and stirring method and using various natural components (betaine, amino acids, sugars, polyalcohol, fatty acids). The synthesized NADES were evaluated for their physicochemical properties, including density, pH, viscosity, conductivity, and polarity, immediately after preparation (time zero) and after a twelve-month storage period to assess their long-term stability. Nuclear Magnetic Resonance and Fourier-transform infrared spectroscopy were utilized to elucidate the structural configurations of the NADES mixtures. These techniques confirmed the formation of hydrogen bonds among the NADES components, except for the one composed only with sugars (glucose, sucrose) and water, thereby validating the successful preparation of the target solvents. Moreover, the impact of dilution on the properties of NADES was investigated, demonstrating that 30 % water addition is optimal, maintaining the desirable properties of the solvent while enhancing certain characteristics like viscosity and density, making them more suitable for extraction purposes and subsequent analysis. The findings from this study offer interesting understanding into the structural and physicochemical properties of NADES, suggesting future applications as green solvents for lab scale extractions but also in various industrial field.